1. Field of the Invention
This invention relates to an exhaust system for an internal combustion engine and, more particularly, to an exhaust system for purifying exhaust gases for an internal combustion engine.
Typically, an exhaust system is endowed with an exhaust gas purifying catalytic converter located in an exhaust pipe downstream from the engine and an exhaust sensor located upstream from the catalytic converter for detecting an oxygen emission level in the exhaust gas. The oxygen emission level is used to feedback control an actual air-fuel ratio of a fuel mixture supplied to the combustion chamber so that it approaches a target air-fuel ratio.
2. Description of Related Art
Devices which are known as "three-way" catalytic converters, including rhodium, are typically used in conjunction with an internal combustion engine to eliminate harmful emissions which can pose health problems from engine exhaust. Particular emissions which ar typically eliminated include carbon monoxide (CO), hydrocarbons (HC) and nitrous oxides (NOx). Such a catalytic converter is located in an exhaust line downstream from an engine combustion chamber. In order to assure functional efficiency of the catalytic converter, the air-fuel ratio of a fuel mixture supplied to the combustion chamber is controlled so that it is maintained at a target air-fuel ratio, e.g., a theoretical air-fuel ratio, of 14.7. In order to conduct the air-fuel ratio control, an exhaust sensor is conventionally located upstream from the catalytic converter. The exhaust sensor provides an output signal which varies in output level before and after an excessive air supply rate is determined to be present. The air supply rate is defined as the ratio of an actual air-fuel ratio to a theoretical air-fuel ratio. An excessive air supply rate is considered to exist when the air supply rate exceeds the value of 1. Restoration of an air-fuel ratio to the theoretical or target air/fuel ratio is performed by decreasing a quantity of fuel supplied when the exhaust sensor detects an air-fuel ratio indicative of a rich fuel mixture. Conversely, such restoration is performed by increasing a quantity of fuel supplied when the exhaust sensor detects an air-fuel ratio indicative of a lean fuel mixture.
A three-way catalyst used in the catalytic converter functionally deteriorates due to impurities in, for example, a leaded gasoline. The impurities typically adhere to the catalytic components even in the insured period of its useful life. Such impurities cause changes in performance during time passage which are taken into consideration in determining the insured period. Functional deterioration of a catalytic converter can be indirectly detected from the number of reversals of output signals from an exhaust sensor, provided downstream from the catalytic converter so as to detect an oxygen level in the exhaust gas, while air-fuel feedback control is conducted based on the output from the exhaust sensor. This is based on the fact that functional deterioration of a catalytic converter is evidenced by the presence of little difference between the concentration of oxygen upstream from the catalytic converter and the concentration of oxygen downstream from the catalytic converter, owing to an reduction in oxygen adsorbability of the catalytic converter. Such a way of detecting deterioration of a catalytic converter in an exhaust system is known from, for instance, Japanese Unexamined Patent Publication No. 63-97,852.
Some exhaust systems of this kind are provided with two catalytic converters located upstream and downstream from one another in order to enhance exhaust purification ability at low temperatures. Such an exhaust system is known from, for instance, Japanese Unexamined Patent Publication No. 64-8,332. In such an exhaust system, in which two catalytic converters are provided upstream and downstream relative to one another, if an exhaust sensor is only placed downstream from the downstream catalytic converter to detect functional deterioration of the catalytic converter on the basis of changes in signal output level from the exhaust sensor, the actual deterioration of each of the catalytic converters cannot be accurately ascertained, owing to differences in quality of the exhaust gases which act on each of the respective upstream and downstream catalytic converters. Consequently, the level of deterioration of each of the catalytic converters will also differ.
Consideration has been given to placing a exhaust sensor between the upstream and downstream catalytic converters to detect functional deterioration of the upstream catalytic converter. However, since greater oxygen adsorption occurs in both the upstream and downstream catalytic converters as compared to an exhaust system equipped with a single catalytic converter, if air-fuel ratio feedback control is conducted based on an output from the exhaust sensor located upstream from the upstream catalytic converter, little change in oxygen pressure in the exhaust gases downstream from the downstream catalytic converter is shown. Accordingly, unless there is a significant functional deterioration of the downstream catalytic converter, the exhaust sensor downstream from the downstream catalytic converter will not have any noticeable change produced in its output signal level. As a result, the accuracy with which functional deterioration is detected is lowered. This is particularly true for the exhaust sensor downstream from the other exhaust sensor.